Interestingly, when we knocked down XAP2 in MCF-7 cells (Fig. 1D–E, MCF-7), we observed a statistically significant up-regulation of endogenous expression of the breast cancer marker gene pS2 in the presence of E2 (Fig. 1A, compare Scr E2 and hsiXAP2 E2). Here, AIP is linked to breast cancer.